Implantable medical devices are available for preventing or treating cardiac arrhythmias by delivering anti-tachycardia pacing therapies and electrical shock therapies for cardioverting or defibrillating the heart. Such a device, commonly known as an implantable cardioverter defibrillator or “ICD”, senses a patient's heart rhythm and classifies the rhythm according to a number of rate zones in order to detect episodes of tachycardia or fibrillation. Rate zone classifications typically include normal sinus rhythm, tachycardia, and fibrillation. Both atrial and ventricular arrhythmias may be detected and treated.
Upon detecting an abnormal rhythm, the ICD delivers an appropriate therapy. Cardiac pacing is delivered in response to the absence of sensed intrinsic depolarizations, referred to as P-waves in the atrium and R-waves in the ventricle, upon the expiration of defined escape intervals. Ventricular fibrillation (VF) is a form of tachycardia that is a serious life-threatening condition and is normally treated by immediately delivering high-energy shock therapy. Termination of VF is normally referred to as “defibrillation.”
Other forms of ventricular tachycardia (VT) may be debilitating, but do not necessarily pose an immediately life-threatening situation. Cardiac output tends to be compromised due to the disorganized contraction of the myocardial tissue resulting in a patient feeling weak, dizzy or even fainting. Ventricular tachycardia may, however, degenerate into a more unstable heart rhythm, leading to ventricular fibrillation. Both VF and VT can result in syncope. Syncope, or fainting, can cause serious injury to the patient, particularly, for example, when the patient is standing or driving a car. A detected VT is generally responded to quickly with either anti-tachycardia pacing therapies or cardioversion shocks. Because VT can often be terminated by anti-tachycardia pacing therapies, these therapies are generally delivered first, because they are less painful to the patient and are followed by high-energy shock therapy only when necessary. Termination of a tachycardia by a shock therapy is commonly referred to as “cardioversion.”
In response to tachycardia detection, a number of tiered therapies may be delivered beginning with anti-tachycardia pacing therapies and escalating to more aggressive shock therapies until the tachycardia is terminated. In modern ICDs, the physician programs the particular therapies into the device ahead of time, and a menu of therapies is typically provided. For example, on initial detection of an atrial or ventricular tachycardia, an anti-tachycardia pacing therapy may be selected and delivered to the chamber in which the tachycardia is diagnosed or to both chambers. On redetection of tachycardia, a more aggressive anti-tachycardia pacing therapy may be scheduled. If repeated attempts at anti-tachycardia pacing therapies fail, a higher energy shock pulse may be selected. Therapies for tachycardia termination may also vary with the rate of the detected tachycardia, with the therapies increasing in aggressiveness as the rate of the detected tachycardia increases. For example, fewer attempts at anti-tachycardia pacing may be undertaken prior to delivery of cardioversion pulses if the rate of the detected tachycardia is above a preset threshold. For an overview of tachycardia detection and treatment therapies reference is made to U.S. Pat. No. 5,545,186 issued to Olson et al., hereby incorporated herein by reference in its entirety. Accurate tachycardia detection and discrimination are important in selecting the appropriate therapy and avoiding the delivery of unnecessary or unsuccessful cardioversion/defibrillation (CV/DF) shocks, which are painful to the patient. Avoiding patient injury due to syncope is also desirable.